Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 20 of 47
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attractive of the intermediate category of grain sizes.
Since the iron and silicon additions in combination appeared to enhance each
other, further binary additives were tried, particularly those involving iron
plus chromium and silicon plus chromium. The outcome of these appraisals
suggested that these three binaries were probably roughly equivalent in
refinement of grain size (at least within the rather crude scale of measure-
ment being employed at the time); whereas binaries involving aluminum did
not appear to offer particular promise in spite of the popularity of an
iron-aluminum alloy preference in the United Kingdom. (It should be
recognized in this respect that the British were using a much higher level
of addition, namely about 300 Fe, 800 Al, than was considered at any time in
these preliminary studies.)
It should be acknowledged that these first explorations of alloy additions
were conducted strictly from an empirical approach since, at the time, there
was no knowledge available concerning the true function fulfilled by the
additives. We had no basis, for instance, to assume that the benefit from
an addition was the result of its presence in solid solution or in its
contribution to the formation of a precipitate phase. The concern at the
time, therefore, was very simple: if the addition appeared to work and if
it did not introduce a serious loss in neutron reactivity, it was considered
favorable, and any exploration of an understanding of the mechanism of its
influence was deferred until a later date.
The final decision regarding the additives to be used in standard production
was therefore quite simply made. Iron at 150 ppm and silicon at 100 ppm were
chosen because: (1) these appeared to provide a grain size entirely
equivalent to the fine-grained ingot uranium that served as a comparison
standard; (2) these were low-cost alloys; and (3) these were favorable in
terms of neutron economy.
Production experience with the 3300-lb. dingots confirmed the effectiveness
of the iron and silicon addition, and it was shown that a favorably close
control of the alloy levels was possible on a day-to-day basis. In actual
operating experience, the variation in either element amounted to + 10 ppm.
There did arise one occasion when the control appeared to have slipped in
that a drift toward moderately coarser grain sizes was observed in the metal
shipped to Hanford. Examination of the trend in chemical composition showed
that the silicon level had been rather consistently on the low side (at
about 90 ppm) although there may have been unrecognized shifts in other
elements whose cumulative effect was not-appreciated; both aluminum and
nickel, for instance, have. been calculated. to exert a significant influence
on grain size even though present in dingot in presumably very minor
quantities. Readjustment of the silicon aim point to the high side was quite
effective in reversing the trend of -grain size drift, and the control of
dingot structure was satisfactory from this point onward. Further evidence
of the acceptability of this type of control was provided by the action of
i. . . . . . .
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Fellows, J. A. Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966., report, January 1, 1966; Weldon Spring, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1033773/m1/20/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.